ABSTRACT Recent analyses of gene mutations in human cancer unexpectedly identified ARHGAP35 encoding p190A RhoGAP (p190A) as a highly mutated gene, i.e. top 30 pan-cancer. Mutations were particularly abundant in adenocarcinomas, including renal cell carcinoma. The mutation spectrum for ARHGAP35 is consistent with a role as tumor suppressor. Moreover the ARHGAP35 locus is located in a region of the genome that frequently is lost in cancer. However, bioinformatics data stop short of establishing the functional consequences of gene mutations. The scope of this proposal is therefore to define oncogenic capacities associated with loss of p190A expression and with naturally occurring ARHGAP35 mutations in human cancer. We have recently obtained results demonstrating that p190A and its paralog p190B - collectively termed p190 here - mediate contact inhibition of proliferation (CIP) in epithelial cells. We moreover conducted a genome wide RNA-seq analysis, and determined that p190 modulates expression of genes regulated by the YAP oncoprotein. YAP is a transcriptional co-activator of TEAD family transcription factors. The activity of YAP is controlled by the Hippo pathway, which is widely implicated in CIP. In Aim 1 we will define mechanisms whereby p190 impacts Hippo signaling and CIP. To this end, we will (i) test whether Rho proteins are required and/or sufficient to promote CIP downstream of p190; (ii) determine if p190 signals through the canonical and/or non-canonical Hippo pathways; (iii) establish if p190 impacts YAP- TEAD-mediated gene expression; and (iv) determine if apparent redundancy between p190A and p190B in epithelial cells is context-dependent. In Aim 2 we will elucidate the function of p190 at adherens junctions (AJs), which play essential roles in promoting CIP through the Hippo pathway. In this aim, we will (i) test a role for p190 to activate the Hippo pathway upon formation of nascent AJs; (ii) establish whether p190 is required for E-cadherin to restore Hippo signaling in carcinoma cells; (iii) define a role for p190 in anoikis, a tumor suppressor mechanism that is modulated by both AJs and the Hippo pathway; and (iv) elucidate if an interaction between p190 and p120- catenin is required for Hippo signaling and CIP. In Aim 3 we will in collaboration with Dr. Gad Getz of the Broad Institute, analyze human tumors with ARHGAP35 mutation for (i) loss-of-heterozygosity; (ii) expression of YAP regulated genes; and (iii) co-mutation and exclusivity data. Next, we will determine the effects of naturally occurring ARHGAP35 mutations on p190A RhoGAP activity, as well as Hippo signaling and CIP. Finally, we will test in vitro and in vivo if expression of exogenous p190A attenuates oncogenic capacities of human cancer cell lines with no or very low levels of endogenous p190A, and if such effects are dependent GAP activity and/or on Hippo signaling. These efforts are essential to elucidate the utility of targeting Rho signaling in future personalized oncology therapy.